Generation of Unidirectionally Propagated Action Potentials - 1

A tripolar nerve cuff applied around a nerve trunk can be used to initiate action potentials (AP) under a central 'cathodic' contact, while arresting AP propagation by a flanking 'anode'. By making one anode carry more current than the other or spacing the escape anode further from the centrally placed cathode, the AP can be arrested at the end carrying the greater current while the other allows the AP to escape. A shorter anode to cathode distance on the left (towards the medial Gastrocnemius muscle) lowers the electrical resistance and allows more current to flow through that anode. This acts as the arresting end of the electrode. The other end of the electrode allows the propagating APs to continue travel. This can ensure arrest of the bidirectionally propagating AP at one end, allowing 'escape' at the other.
Experimental preparation is shown on top left of Figure. R1 is the electromyogram recording site and S1 hook electrode for stimulation. In the bottom, left part of the figure is a detail of an insulated tripolar electrode, shown above as the 'Blocking Cuff'. The dashed lines on the Sciatic nerve represent ring contacts around the nerve inside the insulating silicone rubber sheath. The applied current is unequally divided to the two flanking contacts by using separate stimulators.
In the right side of figure is shown the Medial Gastrocnemius EMG responses to 8 mS square pulses applied through the cuff electrode, with the central contact as the Cathode. There are two distinct responses, a short latency response, which is presumed to have been initiated at the cathode, and the long latency compound AP, which is presumed to have been initiated by an “anodic break” response arising from the left most anode (designated ++) because it is highly correlated with the lagging edge of the 8 ms current pulse. In (a) maximum EMG response at 0.12 mA. In (b to f) growth and then decline of asynchronous activity. (f and g) appearance of post stimulus anodic break response. (h to l) decline and disappearance of direct response (initial deflections) at 1.95 to 3.15 mA. In (k) and (l) a unidirectionally propagating AP, initiated at the cathode, is presumed to have occurred. In(m and n) reappearance of the direct response at higher current amplitudes, but at a slightly shorter latency, presumably arising from a virtual cathode located to the left of the electrode. The “anodic break” response can be eliminated by making the tailing edge of the stimulus pulse decay exponentially.

van den Honert, C. and J. T. Mortimer (1981). “A technique for collision block of peripheral nerve: single stimulus analysis.” IEEE Trans Biomed Engg. 28(5): 373-8.